Typically, a marine steering system includes a steering or helm pump attached to a steering wheel for direction fluid to opposite ends of an actuating cylinder which, in turn, actuates the rudder to effect steering of the boat. These systems are typically closed circuit hydraulic systems.
In closed circuit hydraulic systems, several problems may occur. One is that air is difficult to remove from the oil. Secondly, supercharging may occur due to trapped pressure in the circuit. Further, actuators with unbalanced oil volumes on the supply and return sides, such as a hydraulic cylinder with an output rod at only one end, cannot be used.
The partial return to tank features overcomes these problems. The bleeding of air is aided since air separates from the oil in the portion of the flow return to the tank. By opening the return line to tank each time oil is pumped through the valve, any residual pressure in the system is released. Also, unbalanced actuators can be used since the volume differential is made up or bleed off through the tank port.
Several partial return to tank valves exist in the market today. However, several problems exist in these valves. One such problem is that valves of this type are costly to produce due to the very tight machining tolerances required.
One critical area is lap in the tank port area. Lap in a spool valve is the amount by which the port is opened or closed while the spool is in the center or off position. If the spool completely closes off the tank port, it is overlapped. If the port is slightly opened, it is underlapped. A valve where the spool closes off the port exactly so that the edge of the spool is in line with the edge of the port is called zero lapped.
In a marine steering type partial return to tank valve, the lap on the tank port must be closely controlled. When the spool is touching either check valve, i.e., ready to lift it, it requires an overlap on the pressure side to minimize leakage. The larger the overlap, the lower the leakage and thus less lost motion. On the return side, however, the optimum is zero lapped. If the spool is underlapped in this position, it is actually opening a bypass to tank. Should the pump be reversed, it could cause loss of steering. An overlapped spool allows the return check to open before the tank port is exposed. This results in the system remaining supercharged, although oil is flowing. The operator will sense hard steering until the wheel is surged enough to open the tank port.
A zero lap is, however, very expensive to produce since normal tolerance buildups in the various valve parts will not allow it. As a result, all parts must be match-fitted.
As will be appreciated, a dual operated lock valve prevents motion of the actuator whenever hydraulic pressure is not being supplied to the system while allowing a small flow of oil to or from the reservoir "tank" while it is open. By locking the actuator, rudder drift is prevented. However, when a full size single poppet is opened in an overrunning load condition, the sudden release of pressure causes delivery side pressure to drop and allowing the check to shut again. This process repeats continually as long as flow is being delivered which causes a distinct chatter.
One solution to eliminating chatter is by installing an orifice in the delivery line to hold the pressure on the delivery side of the spool high enough to hold the return check open. Although this is common practice in industrial hydraulics, as shown in U.S. Pat. No. 4,192,338 in the name of Benedict R. Gerulis, it has two significant drawbacks in marine steering. First, the orifice produces a high pressure to hold the spool open, but this also means high steering effort. Secondly, the return check is held fully open which allows the load to run away. Thus, the operator has little control of the amount of motion of the overrunning load since it no longer relates to wheel movement.
Another solution used in industrial hydraulics to eliminate chatter is a dual poppet system where only the smaller poppet is opened on the return side. This system overcomes the load problem, but still suffers from constant high steering loads due to the high pressure drop. Hence, both of the above-mentioned solutions reduce chatter but require a constant high effort in marine steering.